1. Field of the Invention
The present invention relates to an illuminating apparatus for a microscope.
2. Related Background Art
FIG. 7 is a view illustrating the construction of a conventional illuminating apparatus for a microscope.
Beams of light from a light source 101 are first condensed by a condenser lens 102. A cross-sectional area of the light is then narrowed down by an aperture stop 103. The light narrowed down by the aperture stop 103 passes through a relay lens 104 and thereafter travels through a field stop 105 and a field lens 106. The light is subsequently reflected by a half-mirror 107, whereby a real image 112 with respect to the light source 101 and the aperture stop 103 is formed in a rear-side focal position of the objective lens 108. A specimen 109 is irradiated with the light passing through the objective lens 108 at an angular aperture 2α corresponding to an aperture of the aperture stop 103.
When illuminating the specimen 109 with the light at such an angular aperture 2a that a numerical aperture of the objective lens 108 in use is maximized, a maximum resolving-power of the objective lens 108 is obtained, but contrast is lowered. Reversely, the resolving power decreases as the aperture stop 103 is stopped down, but the contrast is enhanced.
Then, it is generally considered that when the aperture stop 103 is stopped down to 70-80% of the numerical aperture of the objective lens 108, the resolving power does not drop down so much, and a well-contrasted illumination is obtained. In general, the objective lens 108 has a larger numerical aperture with a higher magnification. As a result, the angular aperture 2αincreases. When observing a specimen 109 (see FIG. 8) having a minute hole 109a like a contact hole through the high-powered objective lens 108 in, e.g., an LSI exhibiting a larger aspect ratio under such illumination conditions, the illumination is effected by opening the aperture stop 103. Then, as illustrated in FIG. 8, light beams 114b, 114c from the light source 101 correspond to an aperture of the aperture stop 103 and are positioned away from an optical axis L. The light beams 114b, 114c do not reach a bottom of hole 109a, with the result that a difference between an illuminance of the hole 109a and an illuminance of the surface of the specimen 109 becomes too large. The hole 109a is observed as nothing but a black point.
Then, the aperture stop 103 is stopped down; or alternatively, as illustrated in FIG. 9, a pin-hole stop 203 having a pin-hole 203a formed in a masking plate composed of a metal or the like is located at an optical-axis center L instead of the aperture stop 103. With this arrangement, it follows that the specimen 109 is illuminated with light passing through a small angular aperture. This results in a decreased difference between the illuminance of the bottom of the hole 109a and the illuminance of the surface of the specimen 109. The bottom of the hole 109a can be therefore observed.
When illuminating the specimen 109 with light passing through the small angular aperture as shown in FIG. 9, however, the bottom of the hole 109 is observable. But, the following problems arise. The resolving power of the objective lens 108 is deteriorated due to an over stop-down. Besides, an observed only an image exhibits a glare, wherein the contrast is over enhanced.